In the lithography technology, an exposure tool for producing an integrated circuit by transferring a fine circuit pattern onto a wafer has hitherto been widely utilized. With the trend toward a higher degree of integration and a higher performance of an integrated circuit, the refinement of the integrated circuit is advanced, the exposure tool is required to form a circuit pattern with high resolution onto the surface of a wafer at long focal depth, and shortening of the wavelength of an exposure light source is advanced.
The exposure light source is further advancing from conventional g-line (wavelength: 436 nm) and i-line (wavelength: 365 mm) and KrF excimer laser (wavelength: 248 nm), and ArF excimer layer (wavelength: 193 nm) is to be employed.
In the foregoing technology trend, a lithography technique using EUV light (extreme ultra violet light) as an exposure light source is attracting attention, as a lithography technology for producing a semiconductor device having a circuit size finer than 32 to 45 nm. The EUV light means light having a wavelength band of a soft X-ray region or a vacuum ultraviolet region, and specifically means light having a wavelength of from about 0.2 to 100 nm. At this time, use of 13.5 nm is considered as a lithography light source. The exposure principle of the EUV lithography (hereinafter referred to as “EUVL”) is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting therethrough the light having the EUV light energy region, a refracting optical system cannot be used, and a reflecting optical system must be used (See Patent Document 1).
A reflective mask used in EUVL is basically configured with (1) a substrate, (2) a reflective multilayered film formed on the substrate, and (3) an absorber layer formed on the reflective multilayered film. For the reflective multilayered film, a film having a structure that plurality of materials each having different refractive index to a wavelength of exposure light are periodically laminated in nm order is used, and Mo and Si are known as the representative material. Furthermore, Ta and Cr are investigated for the absorber layer.
For the substrate, a material having a low coefficient of thermal expansion is required so as not to generate a strain even under EUV light irradiation, and a glass having a low coefficient of thermal expansion is investigated. In the present description, the glass having a low coefficient of thermal expansion is hereinafter collectively referred to as “low expansion glass” or “ultra-low expansion glass”.
The substrate is produced by cutting those low expansion glass or ultra-low expansion glass material in a desired shape and size; and processing the surface on which the reflective multilayered film is to be formed of the substrate, that is, a main surface of the substrate, such that the surface has an extremely small flatness, specifically, the surface has the flatness of 50 nm or less. For this reason, when the flatness of the main surface of the substrate is measured, the measurement was required to conduct in very high precision with an error within ±10 nm.
On the other hand, regarding side surfaces of the substrate, Patent Document 2 proposes that any of side surfaces, chamfered portions and notched portions of the substrate is mirror polished to a surface roughness Ra of 0.05 μm or less for the purpose of the prevention of generating particles (fine glass pieces) from the side edge surface of the substrate. However, regarding the flatness of the side surface and chamfered portion of the substrate, it was considered that the processing of the ordinary level is sufficient.